Measurement of various flows, for example, slow ones, in a process pipe can be difficult. One example of the measurement of slow flows is the measurement of flows occurring in connection with a bleaching reactor and passing at a rate of about 1 m/min. Problems can arise from measuring devices immersed in the flow disturbing the flow locally, and consequently the measurement result may not be reliable.
When the operation of the bleaching reactor is monitored, an object is to find out the rate of pulp pumped in the reactor from below upwards in the periphery of a reactor tower. This information can be important because the reactor can malfunction, in which the flow in the tower is channeled in such a manner that the pulp flows faster in the center of the reactor as compared with the flow in the border areas. In that case part of the pulp dwells longer in the reactor and can cause variations in bleaching results.
Currently, there are no commercially available measuring devices suitable for monitoring a bleaching reactor. Known technology allows channeling to be examined by adding a tracer in the flow and monitoring its travel time. A drawback with this method is that the tracer ends up in the final product. A further drawback is that the signal produced by the tracer spreads quickly, as the substance mixes in the reactor, and thus the velocity information deteriorates. Instead of a tracer it is also possible to use a temperature pulse, provided by water, whose travel in the reactor is monitored by thermometers mounted in the reactor. A problem with this technology is that the signal can spread and fade quickly as the pulp is mixed.